1. Technical Field
The technology described herein relates generally to wireless networking. More particularly, the technology relates to Multi-User Multiple-Input-Multiple-Output (MU-MIMO) and Orthogonal Frequency Division Multiple Access (OFDMA) communications in a wireless network.
2. Description of the Related Art
Wireless LAN (WLAN) devices are currently being deployed in diverse environments. Some of these environments have large numbers of access points (APs) and non-AP stations in geographically limited areas. In addition, WLAN devices are increasingly required to support a variety of applications such as video, cloud access, and offloading. In particular, video traffic is expected to be the dominant type of traffic in many high efficiency WLAN deployments. With the real-time requirements of some of these applications, WLAN users demand improved performance in delivering their applications, including improved power consumption for battery-operated devices.
A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” A series of standards have been adopted as the WLAN evolved, including IEEE Std 802.11™-2012 (March 2012) (IEEE 802.11n). The IEEE Std 802.11 was subsequently amended by IEEE Std 802.11ae™-2012, IEEE Std 802.11aa™-2012, IEEE Std 802.11ad™-2012, and IEEE Std 802.11ac™-2013 (IEEE 802.11ac).
Recently, an amendment focused on providing a High Efficiency (HE) WLAN in high-density scenarios is being developed by the IEEE 802.11ax task group. The 802.11ax amendment focuses on improving metrics that reflect user experience, such as average per station throughput, the 5th percentile of per station throughput of a group of stations, and area throughput. Improvements may be made to support environments such as wireless corporate offices, outdoor hotspots, dense residential apartments, and stadiums.
An HE WLAN supports Down-Link (DL) and Up-Link (UL) Multi-User (MU) transmissions such as MU Orthogonal Frequency Division Multiple Access (MU OFDMA) transmission, Multi-User Multi-Input-Multi-Output (MU MIMO) transmissions, and MU transmissions that use both OFDMA and MU-MIMO. Collectively, transmissions using OFDMA, MU-MIMO, or both are referred to herein as MU transmissions.
MU communication as define herein are distinguished from other transmissions, such as broadcast transmissions, by the allocation in the MU communication of only a portion of the channel (such as a sub-channel in an OFDMA communication, one or more spatial streams in an MU-MIMO communication, or one or more spatial streams of a sub-channel in a communication using both OFDMA and MU-MIMO) to a particular communication.
Sounding operations are used to identify channel conditions in a wireless network. Identifying the channel conditions permits better utilization of the channel by enabling one or more of higher bit rates, use of a greater number of spatial streams, and more effective utilization of transmission power. However, sounding operations may contribute to the total amount of overhead in a wireless network. Therefore it is advantageous for sounding processes, including sounding processes used for UL MU transmissions, to be efficient.
Protection mechanisms are used in wireless networks to prevent one station's transmissions from interfering with another station's transmissions. For example, a wireless network may use Clear-To-Send (CTS) and Ready-To-Send (RTS) packets to reserve a channel for a particular communication. When MU transmissions are being performed, including cascaded MU transmissions, such protection mechanisms should be efficient in order to increase the effective throughput of the wireless network.
A wireless network performing MU communications may Radio-Frequency (RF) combine all or portions of MU transmissions in the network. RF combining occurs when a plurality of stations simultaneous transmit respective transmissions into a channel, and the transmissions combine in the channel to form a single received transmission, The wireless network may apply scrambling sequences to information being transmitted in order to prevent the transmission of signal patterns which might result in an unwanted regularity in the transmitted signal. When a wireless network uses scrambling sequences, for two or more transmissions to be successfully RF-combined, all of the two or more transmissions must use a same scrambling sequence.